Frequency hopping combined with error protection techniques and interleaving is known to be efficient in combating fast channel fades. This combination often results in a near-Gaussian performance even over hostile Rayleigh channels under the right circumstances. In many cellular networks such as GSM, slow frequency hopping is an optional feature that can be turned on in the network. Each cell or sector is assigned a set of available frequencies over which its transceivers can hop. The carrier frequency of each transceiver is randomly changed every time-division multiple access (TDMA) frame, which lasts about 4.616 msec, based on a list of the available frequencies. Hopping over several frequencies randomizes the occurrence of deep fades, which is commonly referred to as the frequency diversity. This diversity improves the performance of a convolution decoder in the receiver.
If the spacing between the hopping frequencies is sufficiently large (e.g. greater than a coherence bandwidth) then fading envelopes are uncorrelated from one TDMA frame to a next TDMA frame, resulting in a high degree frequency diversity gain. However, a general tendency of GSM systems today is to decrease the separation between the hopping channels to realize tighter reuse patterns such 1/3 and 1/1 reuse. An advantage of this strategy is a high number of hopping frequencies being available in each cell. Yet, the tight frequency reuse also results in low frequency separation between the hopping channels, which decreases the frequency diversity. This problem applies particularly to environments where the coherence bandwidth is large, which may be the case in an indoor or a micro-cellular environment where the time dispersion of the channel is low. If only a small spectrum is available for frequency hopping then a gain associated with frequency hopping in such an environment is relatively small.
For high speed mobiles, the fades are sufficiently uncorrelated from one TDMA frame to the next TDMA frame due to the large Doppler spread. Thus, most of the gain is due to time diversity as compared to frequency diversity. However for pedestrian or stationary mobiles, the gains associated with frequency diversity are predominant provided that the fades from one TDMA frame to the next TDMA frame are sufficiently uncorrelated.